1. Field
Certain aspects of the present disclosure generally relate to wireless communications and, more particularly, to a method for enabling transmission of sounding reference signals for uplink and downlink scheduling.
2. Background
Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) systems and Orthogonal Frequency Division Multiple Access (OFDMA) systems.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-input single-output, multiple-input single-output or a multiple-input multiple-output (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min{NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system supports a time division duplex (TDD) and frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the access point to extract transmit beamforming gain on the forward link when multiple antennas are available at the access point.
In the 3GPP LTE systems, an access terminal may transmit sounding reference signals (SRS) to an access point to provide uplink channel quality information to be used as a basis for scheduling decisions in the access point. The access terminal may transmit cell-specific SRS and UE-specific SRS. Without coordination, transmission of cell-specific SRS and UE-specific SRS may occur at different times. If the access terminal transmits an SRS that collides with other users' data channel transmissions (PUSCH/PUCCH), resulting in performance loss of other users as well as wasting the access terminal's own transit power and causing unnecessary interference. Therefore, it can be beneficial that the access terminal coordinates interaction of cell-specific and UE-specific SRS transmission factors.